Since the introduction of morphine to the clinic as a pain reliever, clinicians have been troubled with the problem of drug addiction. For more than a century, chemists, pharmacologists, and clinicians have strived to find an ideal analgesic with high potency, yet low addictivity. A series of opioids such as meperidine, methadone, and fentanyl were subsequently developed.
However, none of these drugs exert sustained analgesic effects in patients without developing addiction. In Western countries, methadone substitution has been employed for the treatment of drug abuse for some time. Unfortunately, methadone induces significant psychological and physical dependencies. Consequently, patients undergoing such treatment usually convert to methadone dependence during withdrawal from chronic use of morphine, heroin or other opioids (Jaffe, 1990). Therefore the need remains to develop better methods based upon insights into the molecular and cellular mechanisms underlying opioid addiction for treating drug abuse and particularly a means to identify compounds for use as low- or non-addictive analgesics and for suppression of opioid withdrawal symptoms.